(1) Field of the Invention
This invention relates to a method for the surface-hardening treatment of a synthetic resin article and an apparatus for use in carrying out said method.
(2) Description of the Prior Art
Synthetic resins excellent in the molding processability and impact resistance are used in various fields. However, most synthetic resins have problems such that the surfaces of shaped articles of these synthetic resins are not hard, are readily marred and, if mars and scratches are formed on the surfaces, the commercial values of the shaped articles are reduced. These problems are serious particularly in optically transparent materials such as lenses and light-transmitting transparent materials as lamp lenses. It has, therefore, eagerly been desired to impart a scratch resistance to the surfaces of shaped articles.
A method has been proposed and adopted in which a silicone type cured coated film is formed on the surface of a shaped article to improve the surface hardness. As means for forming a uniform coating of a cured film, U.S. Pat. Nos. 3,956,540 and 4,036,168 disclose a method and apparatus for hardening the surface of a shaped article by applying a fluoroethylene polysilicate polymer as the surface-hardening liquid onto the surface of the shaped article. The hardening solution used in this conventional technique is cured by application of heat, and the curing must be carried out in two stages, namely, after the pre-curing step, the surface of the shaped article of a tack-free state must be further treated in the subsequent curing step for imparting thereto the desired mar or abrasion resistance.
Research was made in view of obtaining a synthetic resin shaped article having enhanced surface hardness and abrasion resistance by a method wherein a hardening solution curable with ultraviolet rays is applied onto the surface of a synthetic resin shaped article and whereby a uniform coating can be formed and the coating can be cured in a single curing step.
The surface-hardening method of the present invention falls within the category of the so-called flow-coating method in which a coating composition is flow-coated on an article to be coated. As the flow-coating method adopted in the art of coating, a so-called showering method can be mentioned in which a coating composition is showered onto an article having a complicated shape from many jet nozzles and a so-called curtain-coating method in which a coating composition is fallen down in the form of a film from a long slit of a nozzle onto a substantially plate-shaped article to be coated, which is travelled at a high speed. The former method has problems in that adjustment of the thickness of the coating is difficult, the thickness becomes uneven and a pattern of the flow of the coating composition is readily formed on the resulting cured coating. In the latter method, it is generally necessary to move an article to be coated at a high speed, and a high dimension precision is required for the slit-like orifice in forming a curtain of the liquid coating composition. Furthermore, the latter method has problems in that it is very difficult to form a thin coating and both the front and back surfaces of an article to be coated cannot simultaneously be coated.
For the foregoing reasons, a dip-coating method or a spray-coating method is generally used in various fields for obtaining uniform coatings on an industrial scale. However, the dip-coating method is employed only when the entire surface of an article is coated. Although an article having a complicated shape can be coated according to the spray-coating method, the efficiency of deposition of a coating composition on an article to be coated is low in the spray-coating method and only about 20% to 30% of the coating composition is utilized. Namely, the loss of the coating composition is very large in the spray-coating method. Moreover, the problem of pollution of the working environment and coating equipment is serious in the spray-coating method, and large equipment investment is necessary for preventing environmental pollution.